Outdoor photoluminescence imaging on field-deployed solar cell modules has been conducted to an increasing extent in recent years. Photoluminescence images provide more details about defects than thermal infrared images, while the imaging procedure has the potential to be faster than electroluminescence imaging because it does not require electrical connection to the modules. However, when conducted with sunlight excitation, it is based on lock-in technique, which implies switching between the modules' two operating points. This often results in the need to connect additional electrical equipment dimensioned for the task and leads to production disruption for the investigated modules. The present study demonstrates image acquisition based on the string inverter's ability to sweep the IV curve. The advantage of this approach is twofold. This is a noninvasive method, which only requires imaging apparatus and, therefore, allows for a flexible imaging procedure on the string, module, or cell level without having to take into account additional equipment. Furthermore, photoluminescence images spanning the whole IV curve can be obtained during one sweep, as opposed to obtaining images in two operating points when using lock-in technique. Such image series can be used to investigate the state of the cells and modules by looking at the photoluminescence images acquired on different current extraction levels. This has been done for a healthy string and a broken module.

Spectral imaging techniques are a powerful tool for surveillance and monitoring of degradation mechanisms in PV modules. Luminescence images of solar modules have until recently been predominantly acquired in controlled laboratory settings. Recent attempts have been made in detecting luminescence in daylight. This study aims to present a proof of concept for detection of photoluminescence with sunlight excitation. It enables imaging of several modules in a string as well as several strings simultaneously by changing the operating point through wireless and contactless communication with one or more string inverters. This new approach is validated with the electroluminescence technique as well as with an already established photoluminescence technique under sunlight excitation. Due to the string inverter's reaction time, the change in illumination results in lower image quality. However, with a more up-to-date imaging equipment, this approach is promising for identification of certain degradation mechanisms which can be detected with lower resolution such as large area damage and inactive areas. Further studies are needed to optimize the method and potentially use it in connection with an unmanned aerial vehicle inspection.

Photoluminescence imaging of field-installed photovoltaic modules has the potential to be a high throughput on-site inspection technique. A contribution to this development is a noninvasive photoluminescence imaging method that has been proposed recently. It is based on acquiring images during a progressing current–voltage curve sweep, resulting in a detection of a continuously changing photoluminescence signal. From this follows the necessity to employ an alternative algorithm based on the Pearson correlation coefficient, which will contribute to efficient, unsupervised image processing and, thus, easier (in real-time) implementation. The new algorithm separates photoluminescence from the reflected sunlight, similar to the conventional subtraction algorithm. However, it is robust to varying solar irradiance and can process the photoluminescence signal emitted from multiple asynchronized strings. We predict that it will enable unsupervised real-time surveillance and detection of functional anomalies at a low computational cost. It is also more sensitive at segmenting ground-reflected photoluminescence.

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